Dissociative Recombination in Planetary Ionospheres

Dissociative recombination (DR) is the primary mechanism for electron loss in the ionospheres of Mars and Venus. Surprisingly, O₂⁺ is the most abundant molecular ion, in spite of the low abundance of O₂. As shown in previous laboratory work [1,2], more than 60% of the O₂⁺ produced in the reaction

O⁺ + CO₂ -> O₂⁺ + CO

is vibrationally excited. The result of this vibrational excitation is modification of the branching ratios for the various product channels in the DR reaction

O₂⁺ + e -> O(³P,¹D,¹S) + O(³P,¹D,¹S)

Production of the excited oxygen states, O(¹D,¹S) is a potential source of nightglow emissions [3,4], while production of O(³P) may result in hot atoms with sufficient kinetic energy for exospheric escape [5]. Present knowledge of oxygen DR is limited to the ground vibrational level [6] and to an uncharacterized distribution of excited vibrational levels [7], which showed an increased yield of excited atoms.

We are measuring DR cross section and product distribution of individual vibrational levels at the heavy-ion storage ring in Stockholm (CRYRING) using a high pressure electron-impact ion source to produce controlled vibrational distributions in the molecular ion beam that are fully characterized by observing the products of dissociative charge transfer of the ions in cesium vapor [2].

References

1. D. E. Hunton, A. A. Viggiano, R. A. Morris, J. P. Paulson, D. Smith, and N. G. Adams, "The O⁺ + CO₂ Reaction: New Results and Atmospheric Implications," J. Geophys. Res. 96(A8), 13,881-13,886 (1991).
2. C. W. Walter, P. C. Cosby, and J. R. Peterson, "Rovibrational Product Distributions of O₂⁺ from Reaction of O⁺(⁴S) with CO₂," J. Chem. Phys. 98, 2860-2870 (1993).
3. J. L. Fox, "The Red and Green Lines of Atomic Oxygen in the Nightglow of Venus," Adv. Space Res. 10(5), 31-36 (1990).
4. T. G. Slanger, P. C. Cosby, D. L. Huestis, and T. A. Bida, "Discovery of the Atomic Oxygen Green Line in the Venus Night Airglow," Science 291, 463-465 (2001).
5. J. W. Chamberlain and D. M. Hunten, Theory of Planetary Atmospheres, 2nd Ed. (Academic Press, San Diego, 1987).
6. R. Peverall, S. Rosen, J. R. Peterson, M. Larsson, A. Al-Khalili, L. Vikor, J. Semaniak, R. Bobemkamp, A. Le Padellec, A. N. Maurellis, and W. J. van der Zande, "Dissociative Recombination and Excitation of O₂⁺: Cross Sections, Product Yields, and Implications for Studies of Atmospheric Airglow," J. Chem. Phys. 114, 6679-6689 (2001).
7. D. Kella, L. Vejby-Christensen, P. J. Johnson, H. B. Pederson, and L. H. Andersen, "The Source of Green Light Emission Determined from a Heavy-Ion Storage Ring Experiment," Science 276, 1530 (1997).

Acknowledgements

Principal Investigators and Collaborators

• Philip C. Cosby, SRI <philip.cosby@sri.com>
• David L. Huestis, SRI (E-mail) (CV) (Publications)
• James R. Peterson, SRI
• Wim J. van der Zande, FOM/AMOLF Amsterdam
• Mats Larsson, MSL, U. Stockholm

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• Atmospheric Chemistry and Space Physics at SRI
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